Commercial printing may be done on multi-color continuous web printing presses. Paper rolls are unrolled along a paper path that includes separate stations for each color. An optional dryer may be placed after the final print station. The web is then slit into sheets and stacked.
In ink-jet printing, ink is ejected from a narrow orifice in the direction of a substrate. In one type of ink-jet printing, known as drop-on-demand printing, the ink is ejected in a series of droplets. The droplets may be produced and controlled using a piezoelectric ink-jet head which has a large number of orifices, each of which is separately controllable to selectively eject ink at desired locations, or pixels, of the image. For example, an ink-jet head may have 256 orifices that have spacing for a printing resolution of at least 100 pixels (dots) per inch (dpi) and sometimes far more than that. This dense array of orifices allows complex, highly accurate images to be produced. In high performance print heads, the nozzle openings typically have a diameter of 50 microns or less, e.g., around 25 microns, are separated at a pitch of 25–300 nozzles/inch, have a resolution of 100 to 3000 dpi or more, and provide drop sizes of about 1 to 70 picoliters (pl) or less. Drop ejection frequency is typically 10 kHz or more. A drop-on-demand piezoelectric print head is described in U.S. Pat. No. 4,825,227, the entire contents of which is incorporated herein by reference.
While such dense arrays of orifices produce complex, highly accurate images, image quality can deteriorate if one or more of the orifices become obstructed. For example, a partially obstructed orifice may alter the direction, size, or stability of the droplets. It is important to keep these apertures open and functional to avoid degradation of print quality. It is also important to clean the ink jets quickly without undue equipment down time. Moreover, since position of the ink jets relative to the paper is important, cleaning the heads should not unnecessarily dislocate the print-head module and should return it as close to its original position as possible.